Technical Field
This Patent Document relates generally to capacitive touch sensing, such as for touch/position sensing.
Related Art
Capacitive sensing is a common technology for human interface devices (HID). Example applications are touch and position sensing.
For position sensing, touch position is sensed along a defined sensor track, such as position along a linear track, or radial position around a circular track. The HID capacitive position sensor includes a capacitive sensor configured to define the sensor track, such as a linear slider or circular dial.
One capacitive sensing technique uses projected self-capacitance in which a capacitive sensor (electrode) is disposed on a substrate (such as a PCB) with a ground pattern. The sensor electrode is configured to define the sensor track, with an overlay (coating) that includes position marking/patterning corresponding to the defined sensor track. The sensor electrode is coupled to sensor electronics through an excitation/acquisition channel.
When driven with an excitation signal, the sensor electrode forms a parasitic capacitance with the sensor ground, projecting a sensing E-field through the overlay, and forming a sensing area on the surface of the overlay corresponding to the sensor track defined by the sensor electrode. The capacitive sensor can be configured with an underlying shield that is driven by the sensor electronics to generate a shield E-field with the same polarity and phase as the sensor E-field, concentrating (focusing) the sensor E-field in the sensing direction, and increasing sensitivity.
When a finger (body conductor) touches the sensing area along the sensor track, the projected E-field in the touch/contact area is altered, effectively adding a touch capacitance to the projected capacitance (parasitic capacitance of the sensor electrode) associated with touch position. Total sensor capacitance measured by the sensor electronics at the touch position corresponds to the sum of the projected/parasitic capacitance and the touch capacitance.